Eleven families of phosphodiesterases (PDEs) have been identified but only PDEs in Family I, the Ca2+-calmodulin-dependent phosphodiesterases (CaM-PDEs), which are activated by the Ca2+-calmodulin and have been shown to mediate the calcium and cyclic nucleotide (e.g. cAMP and cGMP) signaling pathways. The three known CaM-PDE genes, PDE1A, PDE1B, and PDE1C, are all expressed in central nervous system tissue. PDE1A is expressed throughout the brain with higher levels of expression in the CA1 to CA3 layers of the hippocampus and cerebellum and at a low level in the striatum. PDE1A is also expressed in the lung and heart. PDE1B is predominately expressed in the striatum, dentate gyrus, olfactory tract and cerebellum, and its expression correlates with brain regions having high levels of dopaminergic innervation.
Although PDE1B is primarily expressed in the central nervous system, it may be detected in the heart. PDE1C is expressed in olfactory epithelium, cerebellar granule cells, striatum, heart, and vascular smooth muscle. Specifically, the major PDE activity in the human cardiac ventricle is PDE1. And PDE1 has been shown to promote human arterial smooth muscle cell proliferation. By virtue of its modulation of cGMP in the monocyte, PDE1 has potential as a hypertrophy regulator. (Circ Res. 2009 Nov. 6; 105(10): 931.) Generally, there is a high abundance of PDE1 isoforms in: cardiac myocytes, vascular endothelial smooth muscle, fibroblast and motor neurons.
Cyclic nucleotide phosphodiesterases downregulate intracellular cAMP and cGMP signaling by hydrolyzing these cyclic nucleotides to their respective inactive 5′-monophosphates (5′AMP and 5′GMP). cGMP is a central intracellular second-messenger regulating numerous cellular functions. In the cardiac myocyte, cGMP mediates effects of nitric oxide and atrial natriuretic peptide, whereas its counterpart, cAMP, mediates catecholamine signaling. Each cyclic nucleotide has a corresponding primary targeted protein kinase, PKA for cAMP, and PKG for cGMP. PKA stimulation is associated with enhanced contractility and can stimulate growth, whereas PKG acts as a brake in the heart, capable of countering cAMP-PKA-contractile stimulation and inhibiting hypertrophy. Importantly, the duration and magnitude of these signaling cascades are determined not only by generation of cyclic nucleotides, but also by their hydrolysis catalyzed by phosphodiesterases (PDEs). PDE regulation is quite potent often suppressing an acute rise in a given cyclic nucleotide back to baseline within seconds to minutes. It is also compartmentalized within the cell, so that specific targeted proteins can be regulated by the same “generic” cyclic nucleotide.
Heart disease is typically a chronic and progressive illness that kills more than 2.4 million Americans each year. There are approximately 500,000 new cases of heart failure per year, with an estimated 5 million patients in the United States alone having this disease. Early intervention is likely to be most effective in preserving cardiac function. It would be most desirable to prevent as well to reverse the morphological, cellular, and molecular remodeling that is associated with heart disease. Some of the most important indicators of cardiac risk are age, hereditary factors, weight, smoking, blood pressure, exercise history, and diabetes. Other indicators of cardiac risk include the subject's lipid profile, which is typically assayed using a blood test, or any other biomarker associated with heart disease or hypertension. Other methods for assaying cardiac risk include, but are not limited to, an EKG stress test, thallium stress test, EKG, CT scan, echocardiogram, magnetic resonance imaging study, non-invasive and invasive arteriogram, and cardiac catheterization.
Pulmonary hypertension (PH or PHT) is an increase in blood pressure in the pulmonary artery, pulmonary vein, and/or pulmonary capillaries. It is a very serious condition, potentially leading to shortness of breath, dizziness, fainting, decreased exercise tolerance, heart failure, pulmonary edema, and death. It can be one of five different groups, classified by the World Health Organization as follows: WHO Group I Pulmonary arterial hypertension (PAH)
a. Idiopathic (IPAH)
b. Familial (FPAH)
c. Associated with other diseases (APAH): collagen vascular disease (e.g. scleroderma), congenital shunts between the systemic and pulmonary circulation, portal hypertension, HIV infection, drugs, toxins, or other diseases or disorder.
d. Associated with venous or capillary disease
Pulmonary arterial hypertension involves the vasoconstriction or tightening of blood vessels connected to and within the lungs. This makes it harder for the heart to pump blood through the lungs, much as it is harder to make water flow through a narrow pipe as opposed to a wide one. Over time, the affected blood vessels become both stiffer and thicker, in a process known as fibrosis. This further increases the blood pressure within the lungs and impairs their blood flow. In addition, the increased workload of the heart causes thickening and enlargement of the right ventricle, making the heart less able to pump blood through the lungs, causing right heart failure. As the blood flowing through the lungs decreases, the left side of the heart receives less blood. This blood may also carry less oxygen than normal. Therefore it becomes more and more difficult for the left side of the heart to pump to supply sufficient oxygen to the rest of the body, especially during physical activity.
WHO Group II—Pulmonary hypertension associated with left heart disease a. Atrial or ventricular disease
b. Valvular disease (e.g. mitral stenosis)
In pulmonary venous hypertension (WHO Group II) there may not be any obstruction to blood flow in the lungs. Instead, the left heart fails to pump blood efficiently out of the heart into the body, leading to pooling of blood in veins leading from the lungs to the left heart (congestive heart failure or CHF). This causes pulmonary edema and pleural effusions. The fluid build-up and damage to the lungs may also lead to hypoxia and consequent vasoconstriction of the pulmonary arteries, so that the pathology may come to resemble that of Group 1 or III.
WHO Group III—Pulmonary hypertension associated with lung diseases and/or hypoxemia a. Chronic obstructive pulmonary disease (COPD), interstitial lung disease (ILD)
b. Sleep-disordered breathing, alveolar hypoventilation
c. Chronic exposure to high altitude
d. Developmental lung abnormalities
In hypoxic pulmonary hypertension (WHO Group II I), the low levels of oxygen may cause vasoconstriction or tightening of pulmonary arteries. This leads to a similar pathophysiology as pulmonary arterial hypertension.
WHO Group IV—Pulmonary hypertension due to chronic thrombotic and/or embolic disease a. Pulmonary embolism in the proximal or distal pulmonary arteries b. Embolization of other matter, such as tumor cells or parasites
In chronic thromboembolic pulmonary hypertension (WHO Group IV), the blood vessels are blocked or narrowed with blood clots. Again, this leads to a similar pathophysiology as pulmonary arterial hypertension.
WHO Group V—Miscellaneous
Treatment of pulmonary hypertension has proven very difficult.
Antihypertensive drugs that work by dilating the peripheral arteries are frequently ineffective on the pulmonary vasculature. For example, calcium channel blockers are effective in only about 5% of patients with IPAH. Left ventricular function can often be improved by the use of diuretics, beta blockers, ACE inhibitors, etc., or by repair/replacement of the mitral valve or aortic valve. Where there is pulmonary arterial hypertension, treatment is more challenging, and may include lifestyle changes, digoxin, diuretics, oral anticoagulants, and oxygen therapy are conventional, but not highly effective. Newer drugs targeting the pulmonary arteries, include endothelin receptor antagonists (e.g., bosentan, sitaxentan, ambrisentan), phosphodiesterase type 5 inhibitors (e.g., sildenafil, tadalafil), prostacyclin derivatives (e.g., epoprostenol, treprostenil, iloprost, beroprost), and soluble guanylate cyclase (sGC) activators (e.g., cinaciguat and riociguat). Surgical approaches to PAH include atrial septostomy to create a communication between the right and left atria, thereby relieving pressure on the right side of the heart, but at the cost of lower oxygen levels in blood (hypoxia); lung transplantation; and pulmonary thromboendarterectomy (PTE) to remove large clots along with the lining of the pulmonary artery. Heart failure and acute myocardial infarction are common and serious conditions frequently associated with thrombosis and/or plaque build-up in the coronary arteries.
Cardiovascular disease or dysfunction may also be associated with diseases or disorders typically thought of as affecting skeletal muscle. One such disease is Duchenne muscular dystrophy (DMD), which is a disorder that primarily affects skeletal muscle development but can also result in cardiac dysfunction and cardiomyopathy. DMD is a recessive X-linked form of muscular dystrophy, affecting around 1 in 3,600 boys, which results in muscle degeneration and eventual death. The disorder is caused by a mutation in the dystrophin gene, located on the human X chromosome, which codes for the protein dystrophin, an important structural component within muscle tissue that provides structural stability to the dystroglycan complex (DGC) of the cell membrane. While both sexes can carry the mutation, females rarely exhibit signs of the disease.
Patients with DMD lack expression of the protein dystrophin as a result of mutations in the X-linked dystrophin gene. Additionally, the loss of dystrophin leads to severe skeletal muscle pathologies as well as cardiomyopathy, which manifests as congestive heart failure and arrhythmias. The absence of a functional dystrophin protein is believed to lead to reduced expression and mis-localization of dystrophin-associated proteins including neuronal nitric oxide (NO) synthase. Disruption of nNOS signaling may result in muscle fatigue and unopposed sympathetic vasoconstriction during exercise, thereby increasing contraction-induced damage in dystrophin-deficient muscles. The loss of normal nNOS signaling during exercise is central to the vascular dysfunction proposed to be an important pathogenic mechanism in DMD.
Currently, there is a largely unmet need for an effective way of treating cardiovascular disease and disorders (e.g. congestive heart disease), and diseases and disorders which may result in cardiac dysfunction or cardiomyopathy (e.g., Duchenne Muscular Dystrophy). Improved therapeutic compositions and methods for the treatment of cardiac conditions and dysfunction are urgently required.